A fuel cell is an electrochemical conversion device that produces electricity by oxidizing a fuel. A fuel cell typically includes an anode, a cathode, and an electrolyte located between the anode and the cathode. A fuel cell system usually includes multiple fuel cells electrically connected to one another in series via interconnects (sometimes collectively referred to as a “fuel cell unit”) and several components configured to provide the fuel to the anodes of the fuel cells and an oxidant to the cathodes of the fuel cells. The oxygen in the oxidant is reduced at the cathode into oxygen ions that diffuse through the electrolyte layers into the anodes. The fuel is oxidized at the anodes, which gives off electrons that flow through an electrical load.
Solid oxide fuel cell (SOFC) systems have an electrolyte formed of solid oxide or ceramic and offer high efficiency and low emissions. Most SOFC anodes include Ni. One disadvantage of SOFC systems is that SOFCs require a relatively high operating temperature—usually between 800 degrees centigrade and 1000 degrees centigrade—to maintain low internal electrical resistance and achieve optimal performance. This high temperature leads to another problem. When the oxygen concentration over the anode of an SOFC exceeds a certain level at temperatures greater than about 300 degrees centigrade to about 400 degrees centigrade (referred to as the “oxidation temperature” in this Background section), anode oxidation may occur. Anode oxidation can eventually lead to anode cracking and declining SOFC system performance depending on the severity of the oxidation.
For instance, when an operator shuts down a typical SOFC system from an operating mode (at which the SOFC system is operating at an operating temperature of about 900 degrees centigrade) to a shut-down mode (at which the SOFC system is at ambient temperature), the SOFC system stops providing fuel to the anodes of the SOFCs. Air takes the place of the fuel. Since the fuel is no longer there to consume the oxygen ions that flow from the cathode to the anode, those oxygen ions accumulate at the anode and, eventually, reach a high enough concentration to oxidize at least some of the Ni in the anodes into NiO. The oxidation of Ni into NiO has two effects. First, the catalytic reactivity and electrical conductivity of the fuel cell anode, both of which are required to drive the electrochemical reaction and produce power, are essentially eliminated. Second, the volume of the anode increases, potentially leading to microstructural (or even macro-structural) damage of the anode, the severity of which depends on the magnitude of the volume change and/or it's frequency.
To prevent these effects, the typical operation of a SOFC system uses a reducing gas that is continually flowed over the anodes during cool down to protect the fuel cell anode from Ni oxidation. This reducing gas is applied from the time that the SOFC fuel flow to the SOFC system is stopped until the SOFC system reaches a temperature that is too low (about 300 C) for the oxidation reaction rate to be significant. The reducing gas, also referred to as transition gas, may be any gas that will reactively consume oxygen.
Similar oxidation effects may occur when the operator re-starts the SOFC system from a shut-down mode at ambient temperature because SOFC fuel is typically not provided to the anodes until the SOFC unit reaches its operating temperature. The absence of SOFC fuel at the anodes during heat-up may result in Ni oxidation of the anodes unless a reducing gas is continually fed to the fuel cell anodes during any heat-up of the SOFC unit above 300 degrees Celsius.
The Ni oxidation of the anodes may be reversible depending on the extent and severity of the oxidation. If an oxidized fuel cell anode is exposed to a transition gas having sufficient hydrogen at an elevated temperature (e.g., the operating temperature), the NiO will be reduced to Ni, restoring the catalytic activity and electrical conductivity of the anode. However, if the Ni oxidation occurs at a sufficiently high temperature, such as that temperature immediately after the initiation of a cool down of the SOFC system (essentially the operating temperature), the anode oxidation has been observed to be more severe and will more likely lead to an irreversible degradation in performance of the fuel cell system. It is hypothesized that oxygen more rapidly and effectively penetrates the Ni particle at higher temperatures, yielding a larger volume change of, and therefore, greater microstructural damage to the anode. Microstructural damage may cause the Ni particles to separate from the membrane material (i.e., the electrolyte), decreasing the available surface area of the reactive triple point, thereby decreasing fuel cell reactivity and overall efficiency power-production efficiency.
Additionally, the effects of the anode oxidation may be amplified by repetitive oxidation and reduction cycles, particularly for cycles having more severe oxidation. Over time, the high-temperature Ni oxidation and corresponding swelling of the anode can cause more severe damage than a single event. This damage may be further amplified by any reduction of the NiO that causes a subsequent shrinking of the anode. Ultimately, these effects can cause macro-structural damage to the anode in the form of visible cracks and delamination (separation) of the fuel cell layers from one another. Macro-structural damage can lead to catastrophic failures of the fuel cell system due to fuel leaks.
One solution to avoid Ni oxidation of the fuel cell anodes, and the resulting fuel cell performance degradation, is to continually provide a reducing gas (also known as a transition gas), typically a mixture of H2 and N2, to the fuel cell anodes whenever the SOFC system is at or above a temperature at which Ni oxidation may occur and SOFC fuel is not being provided to the fuel cell system. The hydrogen gas consumes any oxygen ions that flow from the cathode, thereby preventing the anodes from oxidizing above a temperature of 300° C. Consequently, the fuel cell anodes are never exposed to oxidizing conditions after their initial reduction in this approach. However, this solution is resource intensive and expensive as it requires a large volumes of highly H2-enriched transition gas. The only practical means to supply this gas is with an external system that catalytically converts pipeline natural gas and air to a rich mixture of H2 and CO (CO will also act as a reducing agent by consuming O2).
Another solution utilizes applies a reverse current bias (RCB) to the fuel cell system. The reverse current resulting from the RCB generates O2 and H2 from water and drives O2 back to the cathode, thereby reducing the amount of O2 that collects at the anodes. The RCB applied to the fuel cell system can be controlled to balance the forward reaction (generating H2O from the reaction of H2 with O2) and reverse reactions (breaking up water into H2 and O2 and driving O2 back to the cathode).
These two solutions, providing a transition gas and applying a RCB to the fuel cell system, can be combined to reduce cost associated with the generation of H2-rich transition gas. The amount of H2 generated externally to the fuel cell system and feed to the fuel anodes can be greatly reduced because H2 can be generated using the RCB-driven reaction mentioned above. In theory, it is possible that the anodes may be protected from oxidation without providing an externally generated transition gas, although a finite amount of transition gas will always be required to accommodate air diffusion leakage from cathode to anode and to ensure adequate distribution of reducing gas over the entire fuel cell surface.
With this combined-solution approach, it is feasible to use pressurized bottles to store a reducing gas, that can be in the form of ≤5% H2 in a balance of N2, rather than generating large volumes of highly H2-enriched transition gas. The use of stored transition gas is practical because amount of external-produced hydrogen needed to protect the anodes is reduced and, therefore, the quantity of transition gas to be stored at a plant site would not be unreasonable. A transition has having this lower H2 concentration is also safer in all modes of operation since this gas will always fall below the lowest concentration required to form an explosive mixture when mixed with air. However, the footprint required for this stored gas is still undesirable. Additionally, the stored gas may need to be replaced frequently, e.g., after every start-up and shut down cycle of the power plant.